Powder metal compositions are frequently used to produce metal parts in applications wherein casting, forging or other metal processing techniques are not cost effective. The fabrication of parts using powder metal compositions includes the steps of placing the powder metal composition in the cavity of a mold, pressing the powder metal composition to form a green compact, removing the green compact from the cavity, and firing the green compact to burn out any organic material and densify and consolidate the metal powder into a final part.
Lubricants are employed in pressed powder metallurgy, particularly during the pressing step when the powder is compressed in the cavity to form the green compact. External lubricants, which facilitate the removal of the green compact from the cavity after pressing by ejection, are typically sprayed onto the walls of the cavity prior to filling the cavity with the powder metal composition. Internal lubricants are mixed with the powder metal composition to facilitate slippage of the individual metal particles against each other so that the pressing forces are spread uniformly and the density of the resulting green compact can be made to be as uniform as possible throughout its cross-section.
The use of external lubricants is time-consuming, and it is often difficult to apply a uniform coating of a liquid external lubricant to the cavity walls, particularly when fabricating complex parts. To eliminate the need for external lubricants, some powder metal compositions are formulated to contain an excessive amount of an internal lubricant. In this sense, the phrase “excessive amount” means that the powder metal composition is formulated to contain an amount of an internal lubricant that is greater than would otherwise be necessary to facilitate compaction of the individual metal particles. The use of an excessive amount of an internal lubricant permits the internal lubricant to be in close proximity to the surface of the green compact and provide some lubrication between the green compact and the wall of the mold cavity after pressing. This approach, while effective at diminishing the need for an external lubricant, tends to adversely affect the powder metal composition and metal part making process.
For example, the presence of an excessive amount of internal lubricant in a powder metal composition tends to reduce the flow characteristics of the powder metal composition into the mold cavity, thereby reducing the rate at which the pressing operation can proceed. Furthermore, the presence of an excessive amount of an internal lubricant tends to detrimentally affect the density of the green compact (sometimes referred to as “green density”), because the lubricant takes up volume or space within the mold cavity and interferes with the compressibility of the individual metal particles. At high compaction forces, an excessive amount of an internal lubricant tends to cause delamination and cracking in the green compact, which produces defects in the final part. Furthermore, the presence of an excessive amount of an internal lubricant requires a longer and more complex heating cycle during sintering to remove the larger amount of organic material present. Thus, the use of an excessive amount of an internal lubricant tends to contribute to low final density in the metal part, protracted furnace time, and can lead to the formation of cracks and blisters during firing.